Sampling of hydrocarbons from geological formations

ABSTRACT

A method of separating from a mixture of oil-based drilling fluid and sample of formation hydrocarbon, the liquid component of the drilling fluid, which method comprises treating, e.g. chemically, the mixture in a way that selectively substantially affects the liquid component of the drilling fluid, producing one or more treatment products, while leaving the formation hydrocarbon substantially unaffected; and separating the treatment products from the formation hydrocarbon, by solvent extraction. The liquid component of the drilling fluid conveniently comprises one or more esters (which do not occur in significant quantities in naturally occuring formation hydrocarbons) and which can be hydrolysed to give water-soluble products readily removable from the formation hydrocarbon. The invention is applicable to sampling of formation hydrocarbon using wireline formation testers or sampling tools.

FIELD OF INVENTION

[0001] This invention concerns sampling of hydrocarbons from geologicalformations.

BACKGROUND TO THE INVENTION

[0002] During exploration of a geological formation bearing oil and/orgas (hydrocarbon), and during development and management of extractionof hydrocarbon therefrom, it is important to be able to obtainrepresentative samples of hydrocarbon from the formation. It is known todo this using a down hole wireline formation testers or sampling toolssuch as the Modular Dynamics Formation Tester (MDT) tool ofSchlumberger. For a description of such equipment see Badry, R., Head,E., Morris, C. and Traboulay. I., “New wireline formation testertechniques and applications”, Trans. SPWLA 34^(th) Ann. Logging Symp.,Calgary, June 1993, paper ZZ, and Schlumberger, Wireline FormationTesting and Sampling, pp. 10-1 to 10-25, Schlumberger Wireline andTesting, Houston (1996). The MDT tool allows samples of hydrocarbon tobe captured and maintained at reservoir pressure in sealed containers.The samples are recovered at the surface and analysed to determine theircomposition (in terms of the relative amounts of different hydrocarbons)and their phase (pressure-volume-temperature or PVT) behaviour.

[0003] The acquisition of representative hydrocarbon samples frompermeable formations can be impeded by several problems.

[0004] A major difficulty that commonly occurs when sampling iscontamination by invasion of significant quantities of drilling fluid.This is particularly the case when using oil-based drilling fluids (alsoknown as oil-based muds or OBMs). The base oil in OBMs usually consistsof refined mineral oil, unrefined diesel oil or so-called synthetic oilsuch as poly(alphaolefins) or esters derived from vegetable oils. TheOBM further comprises suspended solids in the form of weighting agent(commonly barite), fluid loss control agent (commonly clay) and possiblyalso drilled solids generated during drilling. The continuous liquidcomponent of OBMs (the filtrate) is completely miscible with theformation hydrocarbons, and it is difficult to distinguish the filtratefrom the formation hydrocarbons. This frequently results in samples offormation hydrocarbon contaminated with filtrate being collected. Suchcontamination results in inaccurate composition and phase behaviourinformation being obtained.

[0005] Another consideration is the length of sampling time. Whilesampling times may be prolonged with the aim of initially pumping outdrilling fluid and contaminated formation hydrocarbon, thus reducing thelikelihood of obtaining contaminated samples, longer sampling timesincrease the risk of the sampling tool and/or cable sticking in thehole. The sticking can be caused by the difference in the pressures inthe drilling fluid column and permeable formations (so-calleddifferential sticking) or by the condition or the geometry of the hole(key seating, swelling/collapsing shale sections etc). There istherefore a direct conflict between the need for extended sampling timesto reduce sample contamination and the need for short sampling times toreduce the risk of sticking.

SUMMARY OF THE INVENTION

[0006] In one aspect, the invention provides a method of separating froma mixture of oil-based drilling fluid (OBM) and a sample of formationhydrocarbon, the liquid component of the drilling fluid (filtrate),which method comprises treating the mixture in a way that selectivelysubstantially affects the liquid component of the drilling fluid,producing one or more treatment products, while leaving the formationhydrocarbon substantially unaffected; and separating the treatmentproduct(s) from the formation hydrocarbon.

[0007] In a further aspect, the invention provides a method ofrecovering a sample of hydrocarbon from a formation, comprising drillinga bore hole into the formation using an oil-based drilling fluid;extracting into the bore hole a sample of fluid comprising hydrocarbonfrom the formation, possibly mixed with the oil-based drilling fluid;treating the sample in a way that selectively substantially affects and,preferentially, selectively affects only, the liquid component of thedrilling fluid, producing one or more treatment products, while leavingthe formation hydrocarbon substantially unaffected; and removing thetreatment product(s) to leave a substantially uncontaminated sample ofhydrocarbon from the formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The invention requires use of a suitable drilling fluid thatyields treatment product or products, which are distinguishable from,and separable from, the formation hydrocarbon. The drilling fluid mustthus be capable of being selectively and differentially treated, to bedistinguishable from the formation hydrocarbon.

[0009] The treatment step typically involves conversion of the liquidcomponent of the drilling fluid into two or more products,distinguishable from the formation hydrocarbon.

[0010] The treatment step may involve one or more stages.

[0011] Typically, the treatment step involves chemical treatment of themixture. In this case, the drilling fluid must be chemically distinctfrom the formation hydrocarbon.

[0012] The chemical treatment preferably involves a catalytic reaction,conveniently catalysed by an enzyme or other similar bio-catalyst.Enzymes (and similar bio-catalysts) have the advantage of oftencatalysing only very specific reactions, enabling targeting of thetreatment to the drilling fluid only. Enzyme-catalysed reactions mayalso occur more rapidly and/or under milder conditions than wouldotherwise be the case. Where an enzyme is used, this is desirably inimmobilised condition, typically immobilised on a solid support in knownmanner. Use of an enzyme in immobilised condition enables ready controlof the location where the enzyme-catalysed reaction occurs, byappropriate positioning of the enzyme, thus assisting the design ofprocessing equipment. Further, immobilisation of enzymes is known toimprove the thermal stability of enzymes and protect them to an extentfrom denaturation that is otherwise frequently observed at hightemperatures, with consequent benefits in the case of treatmentperformed down hole in a wire sampling tool where conditions of hightemperature and pressure are likely to prevail. Enzymes derived fromextromophiles may provide robust catalysts able to function underextreme conditions of pressure and temperature possibly encountered by awireline sampling tool.

[0013] In one preferred embodiment, the liquid component of the drillingfluid comprises one or more esters. Esters do not occur in anysignificant quantities in naturally occuring formation hydrocarbons, andesters are thus chemically distinct from the ingredients of formationhydrocarbons. In this case, the treatment step conveniently compriseshydrolysis of the ester to yield an alcohol and an acid, while leavingthe formation hydrocarbon unaffected. Hydrolysis of an ester can becarried out under acidic or basic conditions as appropriate depending onthe ester, or can be performed enzymically. The products of hydrolysis,possibly after further chemical treatment of the alcohol component, aregenerally water-soluble, and so can be readily separated from theformation hydrocarbon, eg by solvent extraction.

[0014] It is particularly preferred to use an enzyme-catalysed reactionbecause of the specificity and other benefits mentioned above; suitableesterases for hydrolysis of esters are well-known and commerciallyavailable. For example, exposure of a mixture of crude oil and ester toimmobilised pig liver esterase (PLE) (eg as described in Heiss, L.,Gais, H-S., “Polyethylene glycol monomethyl ether-modified pig liveresterase: preparation, characterization and catalysis ofenantioselective hydrolysis in water and acylation in organic solvents”,Tetrahedron Lett., 36, 3833, (1995) and references cited therein) willresult in a mixture of crude oil, alcohol and acid. The latter twocomponents are readily removed by aqueous extraction and, as naturalcrude oils do not contain esters, no components of the reservoirhydrocarbon sample are lost in this process.

[0015] The generalised structure of some aliphatic esters for possibleuse in synthetic oil-based drilling fluids is as follows:

[0016] A wide range of esters satisfying this general formula may beused, with a range of different chain lengths (indicated by n and m),with straight chains (R₁ and R₂=H) or branched chains for the acidcomponent and/or straight chains (R₃ and R₄=H) or branched chains forthe alcohol component. Further, the acid and/or alcohol component mayinclude one or more positions of unsaturation. The ester should beselected to have suitable properties for functioning as a drillingfluid, in terms of chemical stability etc, as is well known in the art.Mixtures of esters may be used.

[0017] As mentioned above, it is known to use esters in oil-baseddrilling fluids. The esters currently used in drilling muds usuallycontain a straight chain attached to a branched alcohol. The branchingmakes the esters more difficult to hydrolyse under normal conditions,providing stability in use. These esters can be hydrolysed underappropriate conditions, e.g. using enzymes, but the resulting alcoholwill usually be sparingly soluble in water. For example, U.S. Pat. No.5,232,910 discloses one drilling fluid comprising isobutyl rape-seed oilester, and another drilling fluid comprising oleic acid isobutyl ester,and U.S. Pat. No. 5,252,554 discloses a drilling fluid comprising lauricacid/n-hexyl ester. Such esters can be used in the method of theinvention.

[0018] The acid product of hydrolysis of an ester is generally highlywater-soluble when in the form of a salt.

[0019] The alcohol product of hydrolysis of an ester may be onlysparingly soluble in water, as is the case with longer and/or branchedchain alcohols. In this event, it may be necessary or appropriatefurther to treat the alcohol, eg. by enzymic or chemical oxidation, toproduce the corresponding acid, to give a water soluble product for easeof separation.

[0020] An example of a typical two-stage treatment step of this sort isas follows:

[0021] It may be preferred to use an ester having a short chain(branched or linear) alcohol component (e.g. iso-butanol rather than2-ethyl hexanol as in the reaction scheme above) to yield awater-soluble alcohol on hydrolysis.

[0022] A further possibility is to use esters in which the acidcomponent is branched, with the alcohol component being branched orlinear, as this will allow the use of shorter chain esters which areresistant to hydrolysis under normal drilling conditions yet whichhydrolyse (e.g. on exposure to a suitable esterase) to yield two, smallwater soluble species, readily separable from formation hydrocarbon. Theuse of α-branched carboxylic acids, branched at the a carbon adjacent tothe carbon of the carbonyl group, for example, may confer someadvantages.

[0023] It is well known that the esters formed from α-branchedcarboxylic acids exhibit marked resistance to alkaline hydrolysis. Theincreased hydrolytic stability of esters formed with α-branchedcarboxylic acids may enable lower molecular weight acids to be used andwith a subsequent reduction in the viscosity of the drilling fluid.

[0024] In the method of the invention it is thus proposed particularlyto use an ester of an α-branched acid, preferably with a linear alcohol.Such esters have good stability to hydrolysis under typical down-holeconditions of use, yet are hydrolysable under appropriate conditions toyield water-soluble products. The α-branched acid is preferablyrelatively short-chained, having up to 12 carbon atoms. This approachalso has the added advantage of increasing the water solubility of theacid salt generated compared to that of the non-branched acid. Theexample shown below is of 2-methyl oleic acid-methyl ester.

[0025] A further possible approach is to use esters of polyols such asdiols and triols. The use of a triglyceride as an ester for drillingmuds has not previously been proposed. These compounds are extremelyhydrophilic and hydrolytically stable. They can, however, be hydrolysede.g. by porcine pancreatic lipase (see Faber, K Biotransformations inOrganic Chemistry, 3^(rd) edition, 1996, and references cited therein)to give a highly water soluble triol and acid. This reaction isfacilitated using biphasic media. An example is illustrated below.

[0026] It may be convenient to use a mixture of esters, eg based onnaturally occurring vegetable oils having a range of different acidcomponents of different chain length.

[0027] The liquid component of the drilling fluid may alternatively oradditionally comprise other hydrolysable chemical species, eg amides,acetals and ortho-esters. Non-hydrolysable chemical species mayalternatively be used, eg poly(alphaolefins). Poly(alphaolefins) arehydrocarbons manufactured by catalytic oligomerization of linearα-olefins having six or more, usually 10 carbon atoms.Poly(alphaolefins) have terminal unsaturation not generally found insignificant quantities in naturally occurring hydrocarbons present information hydrocarbon. Terminal double bonds of the form —CH═CH₂ havebeen identified in some crude oils (see Curiale, J. A. and Frolov, E.B., “Occurrence and origins of olefins in crude oils. A criticalreview”, Organic Geochemistry, 29, 397-408 (1998)). The olefin contentof crude oils is typically in the range 0.02-10 weight percent, althoughthis upper concentration is rarely achieved, and 1-2 weight percent ismore common. Generally the olefin content of a crude oil increases asthe age of the reservoir increases. The terminal olefin content of mostcrude oils is low and the loss of these olefins during the extraction ofolefin-based drilling fluid will generally have a negligible effect onthe phase behaviour or composition of the crude oil sample. Theseparated treatment products can be analysed to identify the presence ofany compounds derived from olefins originally in the crude oil sample.If an abnormally high content of naturally-occurring terminal olefins inthe reservoir hydrocarbon samples is suspected before sampling, anester- or other non-olefin-based oil can be used in the drilling fluid.With all such drilling fluids, the treatment step must in each case besuitably tailored to yield one or more products separable from theformation hydrocarbon.

[0028] For example, with poly(alphaolefins) an enzymic or other chemicalprocess may be used that converts such compounds to water-solublematerials. For a discussion of enzymic processes, see the Faberreference mentioned above. For other chemical approaches see March, J.,Advanced Organic Chemistry, 4^(th) edition, pp. 336-340,Wiley-Interscience, New York (1992). Options for this process includeoxidative-cleavage of the olefins and subsequent further oxidation tothe corresponding diol or carboxylic acid, both of which are soluble inaqueous media. This approach could be achieved using a variety ofreaction schemes. Two possible routes are shown schematically below:

[0029] With this approach it is important to note that the terminaldouble bonds found only in the drilling fluid filtrate can bedistinguished from the non-terminal double bonds found in thenaturally-occurring hydrocarbons in oil samples.

[0030] Amides, acetals and ortho-esters are functionalities that can behydrolysed to give alcohols and polar functionalities, which are usuallywater soluble. Chemical or enzymatic hydrolysis of these molecules givesproducts that can be extracted into aqueous media. An example is shownbelow for an ortho-ester.

[0031] The separation step conveniently uses a physical separationtechnique. Appropriate techniques can be readily determined and selectedhaving regard to the nature and properties of the treatment product orproducts. One convenient separation technique is solvent extraction.Where the treatment product or products are water soluble, as discussedabove, they can be readily removed from the water-insoluble formationhydrocarbon in the mixture under treatment, by extraction into water inknown manner. Solvent extraction using other solvents or mixtures ofsolvents may be adopted in some cases: for example, poorly water-solublealcohol products of hydrolysis of esters, as discussed above, may beextracted in known manner into a mixture of water and alcohol.

[0032] The invention is applicable to sampling of formation hydrocarbonusing wireline formation testers or sampling tools such asSchlumberger's MDT tool, as discussed above. The tool is used inconventional manner to obtain a sample of what is believed to beformation hydrocarbon, possibly contaminated with oil-based drillingfluid. Suspended solids will generally be initially removed inconventional manner, usually by physical filtration methods, to leavethe formation hydrocarbon possibly mixed with drilling fluid filtrate.By processing this mixture by the method of the invention, the filtrate,if present, can be removed, leaving a substantially pure sample offormation hydrocarbon ready for analysis.

[0033] The processing by the method of the invention can be performed insitu in the tool under downhole conditions or at the surface under lessdemanding conditions, eg in a laboratory.

[0034] On-line sensors can be used to monitor the process. For example,the optical fluid analyser (OFA) of the tool can be used to monitor theprocess in the wireline sampling tool, or a similar spectrometer orother sensor can be used to monitor the process in a laboratory. Anon-line sensing device will allow the changes in the mixture to bemonitored before and after treatment and separation. This continualtreatment/separation/monitoring process will allow the clean-up processto be carried out at an optimal rate, and by monitoring the process itis possible to determine when the clean-up process is complete. When thesensor has determined that there is no further change in the compositionof the mixture, this indicates that the clean-up process is complete;the formation hydrocarbon sample can then be considered as contaminantfree.

[0035] The ability to remove contaminating drilling fluid filtrate froma formation hydrocarbon sample obviates the need for extended samplingtimes, thereby reducing the risk of tool sticking.

[0036] In a further aspect, the invention provides a method ofrecovering a sample of hydrocarbon from a formation, comprising drillinga bore hole into the formation using an oil-based drilling fluid;extracting into the bore hole a sample of fluid comprising hydrocarbonfrom the formation, possibly mixed with the oil-based drilling fluid;treating the sample in a way that selectively substantially affects and,preferentially, selectively affects only, the liquid component of thedrilling fluid, producing one or more treatment products, while leavingthe formation hydrocarbon substantially unaffected; and removing thetreatment product(s) to leave a substantially uncontaminated sample ofhydrocarbon from the formation.

[0037] Sample extraction is conveniently carried out using a downholewireline formation tester or sampling tool such as a MDT tool fromSchlumberger.

[0038] The invention will be further described, by way of illustration,in the following Example.

EXAMPLE

[0039] A mixture of formation hydrocarbon and the oil-soluble esteriso-butyl oleate was prepared to simulate a sample of hydrocarboncontaminated with oil-based drilling fluid filtrate comprising iso-butyloleate. The mixture was circulated over a surface bearing immobilisedesterase, eg pig liver esterase as referred to above. This results inhydrolysis of the ester to give oleic acid and isobutanol, while leavingthe formation hydrocarbon unaffected. The mixture was then contactedwith a slightly basic aqueous solution (containing potassium carbonate,for example), into which dissolve the water-soluble alcohol iso-butanol,and the water soluble metal salt of the oleic acid. The aqueous solutionis separated from the immiscible formation hydrocarbon, leaving a sampleof formation hydrocarbon free from contamination. The process isillustrated in the following reaction scheme:

[0040] This approach can be applied during extraction of hydrocarbonsamples from a formation, eg by using a Schlumberger Modular DynamicsFormation Tester (MDT) tool, using an oil-based drilling fluid in whichthe oil is iso-butyl oleate.

[0041] The hydrolysis/separation process can be performed in thewireline sampling tool at reservoir conditions, or in a laboratory atambient temperature. On-line sensors can be used to monitor the process.For example, the optical fluid analyser (OFA) of the tool can be used tomonitor the process in the wireline sampling tool, or a similarspectrometer or other sensor can be used to monitor the laboratoryprocess. An on-line sensing device will allow the changes in the mixtureto be monitored before and after contact with the immobilised enzyme.This continual extraction/monitoring process will allow the clean-upprocess to occur at an optimal rate and this process can be monitored todetermine when the process is complete. When the sensor has determinedthat the process is complete (i.e., there is no further change is thecomposition of the mixture), the sample can then be considered ascontaminant free.

1. A method of separating from a mixture of oil-based drilling fluid anda sample of formation hydrocarbon, the liquid component of the drillingfluid, which method comprises treating the mixture in a way thatselectively substantially affects the liquid component of the drillingfluid, producing one or more treatment products, while leaving theformation hydrocarbon substantially unaffected; and separating thetreatment product(s) from the formation hydrocarbon.
 2. A methodaccording to claim 1, wherein the treatment step involves chemicaltreatment of the mixture.
 3. A method according to claim 2, wherein thechemical treatment involves an enzyme-catalysed reaction.
 4. A methodaccording to claim 3, wherein the enzyme is in immobilised condition. 5.A method according to any one of the preceding claims, wherein theliquid component of the drilling fluid comprises one or more esters. 6.A method according to claim 5, wherein the drilling fluid comprises anester of an α-branched acid.
 7. A method according to claim 5 or claim6, wherein the treatment step comprises hydrolysis of the ester to yieldan alcohol and an acid.
 8. A method according to any one of thepreceding claims, wherein the liquid component of the drilling fluidcomprises poly(alphaolefins).
 9. A method according to claim 8, whereinthe treatment step comprises one or more oxidations of thepoly(alphaolefins) in order to obtain diols or carboxylic acids.
 10. Amethod according to claim 9, wherein the oxidation(s) is doneenzymatically or chemically.
 11. A method according to anyone of thepreceding claims, wherein the separation step uses a physical separationtechnique.
 12. A method according to claim 11, wherein the separationstep comprises solvent extraction.
 13. A method of recovering a sampleof hydrocarbon from a formation, comprising drilling a bore hole intothe formation using an oil-based drilling fluid; extracting into thebore hole a sample of fluid comprising hydrocarbon from the formation,possibly mixed with the oil-based drilling fluid; treating the sample ina way that selectively substantially affects the liquid component of thedrilling fluid, producing one or more treatment products, while leavingthe formation hydrocarbon substantially unaffected; and removing thetreatment product(s) to leave a substantially uncontaminated sample ofhydrocarbon from the formation.
 14. A method according to claim 13,wherein sample extraction is carried out using a downhole wirelineformation tester or sampling tool.